The disclosure relates to methods and systems for analyzing an image of a geological formation, such as a borehole image.
One of the main tasks in geology is to describe and classify geological layers, in order to understand the subsurface structure and the geological history.
Reservoir evaluation in the oil industry aims at creating an accurate reservoir model. For this purpose, wells are drilled and the geological facies is evaluated. The term “facies” (or “lithofacies”) denotes overall characteristics of such visual properties. The spatial analysis of facies in a geological field allows linking the distant layers, and identifying the geological structure.
Field geologists describe geological layers by observing rock samples by naked eye, or under microscope and classify the layers. They observe visual properties such as color, geometrical properties including grain size, because the origin of the rock and its history necessarily have an impact on its current visual properties (for example, color derives from mineral composition).
Conventionally, type of rock in the subsurface is also often characterized by electrofacies, which are properties obtained through geophysical logs such as resistivity and gamma ray. The definition of electrofacies deviates from the original definition of facies, because electrofacies does not necessarily differentiates “visible” rock properties. Electrofacies obtained from conventional logs is generally not as descriptive as facies in geology but it is linked to the facies (e.g. higher resistivity of rock is associated to larger grain size).
Borehole image is a type of log, which provides a high-resolution map of a specific property around borehole wall (for instance, the resistivity). Borehole images are often textured and accordingly provide both characteristics of electrofacies by the value of the property at each location, and the characteristics of facies by the textural properties. Borehole image, accordingly, is more descriptive to define rock properties than non-image type of logs.
In the present application, the term “texture” represents a pattern that is spatially homogeneous and consist of repeated elements, often subject to some randomization in their location, size, color, orientation, etc.
Currently, borehole images are analyzed by interpreters that are highly qualified geologists and are able to define facies based on the image texture. This task requires expertise and experience, and more importantly, it is time consuming.